Robust control of internal combustion engine speed, especially speed at or near idle, persists as a particularly challenging engine control issue. Rejection of the significant engine load disturbances that frequently occur involves adjustment of the engine operating point. The adjustment must respond rapidly and in proportion to the load disturbance for proper rejection. Improper rejection can result in perceptible and annoying engine speed variation. Accordingly, it is desirable to rapidly and proportionately respond to engine load disturbances, especially at idle.
Prediction of changes in engine speed and engine torque before such changes become manifest is generally known in the art to increase the amount of time available to prepare and deliver the described appropriate adjustment to the engine operating point. Situational feedforward control has been proposed in which changes in engine speed are predicted by monitoring the status of the wide variety of actuators and engine parameters that tend to impact engine speed stability.
The difficulties with situational feedforward control are numerous. For example, a large number of signal and actuator status combinations must be calibrated and periodically analyzed to ascertain the overall impact thereof on engine speed. Compromises may be required in the sophistication of the control in order to process all of the combinations, such as quantizing or altogether ignoring the magnitude of the load change corresponding to the combinations. Common treatment for a wide range of load changes follows from such compromises, reducing the robustness of the engine speed control. Furthermore, some of the load disturbances addressed in situational feedforward control, such as load changes caused by power steering cramp, are very difficult to accurately estimate or measure. Still further, preclusively high sampling rates may be required to properly monitor rapidly changing signals or actuators.
Feedback control has likewise been applied in engine speed control, in which sensed changes in load or engine speed or engine torque may be addressed. However, the reliance of such control on sensing of the control parameters themselves can unacceptably reduce the responsiveness of the control.
Accordingly, what is needed is an engine speed control improvement that avoids the difficulties of situational feedforward control and provides for a more responsive feedback control of engine speed.